I always cringe when people look at buying a house with A)No money down or B)depleting all their cash for down payment.
The average homeowner spends 10-15% of their purchase price of a new home, on general household expenses and items the VERY FIRST YEAR they move in. As mentioned above, you have lawn mowers, weed eaters, garden hoses, paint, window treatments, grass seed, sprinklers, new furniture (you can't move that old couch into the new house

...) the list goes on and on. The issue you will have if you attempt to buy a house when you have no savings is that you will do just as you did when you got married... You will spend money you don't have and put it on credit cards. The next thing you know, you will owe a couple thousand dollars on a credit card or two and not even know how the balances got that high. Then you have a house payment, car payment, credit card payment, etc. and you start putting basic expenses on the credit card. trust me, I see it all the time.
I think home ownership is great but many young couples get caught up in the hype of homeownership because they are "throwing" rent money away. In reality, if you finance 100%, do a 30 year mortgage, your principal reduction will be minimal and likely less than the annual maintenance on your home.
Again, home ownership is great, just don't rush into it and cause yourself to dig a hole you can't get out of. Sometimes renting, especially while young, makes way more sense than people give credit.
Back to your original post question, (after my opinion) if you want to borrow for your down payment, it should be OK. The 401K loan is likely your best option. The mortgage lender will not even have to count it against you as debt. As mentioned though, you may be able to withdraw money (not a loan) without penalty. there is no risk in this as you never paid taxes on this so it's no different than your boss giving you a big bonus check. You will simply pay taxes on it at the time of withdrawal.
Just make sure the timing is right for the purchase and you don't let the stress of a new home get in the way of enjoying your first few years of marriage.